diff --git a/src/addons/filter/mathjaxloader/services/handlers/mathjaxloader.ts b/src/addons/filter/mathjaxloader/services/handlers/mathjaxloader.ts
index 40f83f4ba..ad398a11d 100644
--- a/src/addons/filter/mathjaxloader/services/handlers/mathjaxloader.ts
+++ b/src/addons/filter/mathjaxloader/services/handlers/mathjaxloader.ts
@@ -190,7 +190,7 @@ export class AddonFilterMathJaxLoaderHandlerService extends CoreFilterDefaultHan
     protected insertSpan(text: string, start: number, end: number): string {
         return CoreTextUtils.substrReplace(
             text,
-            '<span class="nolink">' + text.substring(start, end - start + 1) + '</span>',
+            '<span class="nolink">' + text.substring(start, end + 1) + '</span>',
             start,
             end - start + 1,
         );
diff --git a/src/addons/mod/lesson/services/lesson.ts b/src/addons/mod/lesson/services/lesson.ts
index 7d1418032..4a33b1617 100644
--- a/src/addons/mod/lesson/services/lesson.ts
+++ b/src/addons/mod/lesson/services/lesson.ts
@@ -760,7 +760,7 @@ export class AddonModLessonProvider {
             let ignoreCase = '';
 
             if (useRegExp) {
-                if (expectedAnswer.substring(-2) == '/i') {
+                if (expectedAnswer.slice(-2) == '/i') {
                     expectedAnswer = expectedAnswer.substring(0, expectedAnswer.length - 2);
                     ignoreCase = 'i';
                 }
diff --git a/src/addons/mod/scorm/classes/data-model-12.ts b/src/addons/mod/scorm/classes/data-model-12.ts
index 97889bfdb..8c82d6876 100644
--- a/src/addons/mod/scorm/classes/data-model-12.ts
+++ b/src/addons/mod/scorm/classes/data-model-12.ts
@@ -748,7 +748,7 @@ export class AddonModScormDataModel12 {
                     const childrenStr = '._children';
                     const countStr = '._count';
 
-                    if (elementModel.substring(elementModel.length - childrenStr.length, elementModel.length) == childrenStr) {
+                    if (elementModel.substring(elementModel.length - childrenStr.length) == childrenStr) {
                         const parentModel = elementModel.substring(0, elementModel.length - childrenStr.length);
 
                         if (this.dataModel[this.scoId][parentModel] !== undefined) {
@@ -756,7 +756,7 @@ export class AddonModScormDataModel12 {
                         } else {
                             this.errorCode = '201';
                         }
-                    } else if (elementModel.substring(elementModel.length - countStr.length, elementModel.length) == countStr) {
+                    } else if (elementModel.substring(elementModel.length - countStr.length) == countStr) {
                         const parentModel = elementModel.substring(0, elementModel.length - countStr.length);
 
                         if (this.dataModel[this.scoId][parentModel] !== undefined) {
diff --git a/src/addons/mod/url/services/url.ts b/src/addons/mod/url/services/url.ts
index 86a2ad5bb..332663f1a 100644
--- a/src/addons/mod/url/services/url.ts
+++ b/src/addons/mod/url/services/url.ts
@@ -68,7 +68,7 @@ export class AddonModUrlProvider {
         const download = ['application/zip', 'application/x-tar', 'application/g-zip', 'application/pdf', 'text/html'];
         let mimetype = CoreMimetypeUtils.getMimeType(extension);
 
-        if (url.externalurl.indexOf('.php') != -1 || url.externalurl.substring(-1) === '/' ||
+        if (url.externalurl.indexOf('.php') != -1 || url.externalurl.slice(-1) === '/' ||
                 (url.externalurl.indexOf('//') != -1 && url.externalurl.match(/\//g)?.length == 2)) {
             // Seems to be a web, use HTML mimetype.
             mimetype = 'text/html';
@@ -158,7 +158,7 @@ export class AddonModUrlProvider {
         const matches = url.match(/\//g);
         const extension = CoreMimetypeUtils.getFileExtension(url);
 
-        if (!matches || matches.length < 3 || url.substring(-1) === '/' || extension == 'php') {
+        if (!matches || matches.length < 3 || url.slice(-1) === '/' || extension == 'php') {
             // Use default icon.
             return '';
         }
diff --git a/src/core/singletons/colors.ts b/src/core/singletons/colors.ts
index 368b3e99d..40c2cdffe 100644
--- a/src/core/singletons/colors.ts
+++ b/src/core/singletons/colors.ts
@@ -123,8 +123,8 @@ export class CoreColors {
 
         return {
             red: parseInt(color.substring(0, 2), 16),
-            green: parseInt(color.substring(2, 2), 16),
-            blue: parseInt(color.substring(4, 2), 16),
+            green: parseInt(color.substring(2, 4), 16),
+            blue: parseInt(color.substring(4, 6), 16),
         };
 
     }
diff --git a/src/core/singletons/locutus.ts b/src/core/singletons/locutus.ts
index ac9b5bc3e..be8b74d97 100644
--- a/src/core/singletons/locutus.ts
+++ b/src/core/singletons/locutus.ts
@@ -123,7 +123,7 @@ function readBytes (str, len, escapedString = false) {
     const isLowSurrogate = code >= 0xdc00 && code <= 0xdfff
 
     if (escapedString && chr === '\\') {
-      chr = String.fromCharCode(parseInt(str.substring(c + 1, 2), 16))
+      chr = String.fromCharCode(parseInt(str.substring(c + 1, c + 3), 16))
       escapedChars++
 
       // each escaped sequence is 3 characters. Go 2 chars ahead.